A fixing device for fixing a toner image is used in an electrophotographic image forming device, for example, a printer, a copying machine, and a facsimile machine, as a fixing device. A known fixing device using an endless belt (a fixing belt) is effective in power-saving or improving a rise time speed in contrast with the conventional heat roller method. For example, Japanese laid-open patent application No. 2007-322888 discloses a fixing device using such a fixing belt.
In a fixing device using a fixing belt, a fixing roller is arranged in a position that is counter to a pressure roller and inside a loop of the fixing belt, and a nipping part is formed at the position where the fixing roller is pressed by the pressure roller. The fixing belt is placed in tension by the fixing roller and a support. When the pressure roller is rotated by a driving source, the fixing belt and the fixing roller are driven to rotate by the frictional force between the pressure roller and the fixing belt, and the frictional force between the fixing belt and the fixing roller. A medium is fed and pressed through the nipping part, and the unfixed toner on the medium is fixed as an image by heat and pressure, in the condition that a temperature of the fixing belt is heat-controlled by a heat source to a predetermined value.
However, in the conventional fixing device using the fixing belt mentioned above, heat is transferred to the medium from a part (a medium-contacting part) in contact with the medium on the fixing belt, and the temperature of the part is lowered accordingly. On the other hand, a part of the fixing belt (a non-medium-contacting part) that does not contact the medium maintains a relatively high temperature since heat is not transferred to the medium from such part. The heat conductivity of a fixing belt is low and a significant amount of heat is not transferred from the non-medium-contacting part to the medium-contacting part in the belt itself. Therefore, a difference in temperature occurs between the medium-contacting part and the non-medium-contacting part on the fixing belt.
As a result, when an image is fixed to a narrow-width medium (NWM), which is much narrower than the fixing belt, the non-medium-contacting part becomes relatively warmer since it is heated without losing heat to the medium. When an image is fixed to a wide-width medium (WWM) just after that, the fixing temperature differs between the medium-contacting part and the non-medium-contacting part, which results in unevenness in the fixing of the toner.
The present invention is made in view of the problem mentioned above, and it is the object to provide a fixing device that makes the difference in temperature between a medium-contacting part and a non-medium-contacting part as small as possible, even when an image is fixed to an NWM followed by a WWM, which improves the uniformity of the image fixing of the WWM that follows the NWM.